Problem: Simplify the following expression: $p = \dfrac{r}{9} - \dfrac{3r}{4}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9$ and $4$ $\lcm(9, 4) = 36$ $ p = \dfrac{4}{4} \cdot \dfrac{r}{9} - \dfrac{9}{9} \cdot \dfrac{3r}{4} $ $p = \dfrac{4r}{36} - \dfrac{27r}{36}$ $p = \dfrac{4r -27r}{36}$ $p = \dfrac{-23r}{36}$